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Barcan formula
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== Converse Barcan formula == The converse Barcan formula is: :<math>\Box \forall x Fx \rightarrow \forall x \Box Fx</math>. It is equivalent to :<math>\exists x\Diamond Fx\to\Diamond\exists xFx</math>. If a frame is based on a symmetric [[accessibility relation]], then the Barcan formula will be valid in the frame if, and only if, the converse Barcan formula is valid in the frame. It states that domains cannot shrink as one moves to accessible worlds, i.e. that individuals cannot cease to exist. The converse Barcan formula is taken to be more plausible than the Barcan formula.
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